The process of manufacturing of dissolving pulps results in a byproduct stream comprised of unbleached pulp fines. The principal source of this byproduct is the sidehill screening of pulp. In some pulp processes sidehill screening is followed by a dissolved air flotation process (DAF). Dissolved air flotation process is a fractionation technique used in the pulp and paper industry to remove pulp fines and other particulates present in paper machine white water or other process waters. The process usually involves the use of micro-bubbles as a carrier phase. This process is also widely used in the treatment of industrial wastewaters for the removal or separation of, for example, macromolecules, chemicals, microorganisms, colloids, fibers, and fines. (A. Féris and J. Rubio (1999), Dissolved Air Flotation (DAF) Performance at Low Saturation Pressures, Filtration & Separation, vol. 36:9, November 1999, pages 61-65). In the DAF process an adjustable stream of the process water is saturated with air before entering the flotation chamber where the pressure is reduced to atmospheric pressure. Mixing of nonsaturated process water and air-saturated water can be done before or after entering the chamber. When the pressure of the air-saturated water is reduced billions of micro-bubbles in the range of 30-100 μm are formed. [A. L. Macfarlane, R. Prestidge, M. M. Farid and J. J. J. Chen (2007). These bubbles adhere to the suspended solids in the feed water stream. Suspended particles rise to the surface due to buoyancy forces, where they are skimmed off or float over a ridge in the chamber. The suspended particles usually comprise fines—cellulosic microfibers which typically have a length of no greater than about 200 μm, and other organic materials usually in association with inorganic particulate materials. The particles in such streams have, in some cases, proven difficult to dewater, in addition these waste streams are environmentally and economically undesirable to discharge.
The unbleached pulp fines utilized in the present invention, referred herein as simply pulp fines, are small particles that exist in all types of pulp. The main sources of pulp fines are 1) ray cells and axial parenchyma cells originating in wood; 2) fiber fragments from the wood chipping process; and 3) damaged fiber fragments from cooking and bleaching. The pulp fines of the invention can be isolated from paper machine white water or other pulp or paper industry processing streams using screening, such as sidehill screening, and/or DAF separation. As indicated above, the pulp fines utilized in the invention are unbleached cellulosic material, which differs from market wood pulp in that it has higher contents of lignin, hemicelluloses, metal, extractives, and non-cellulosic origin polymers.
The collected pulp fines are subsequently washed to remove organic materials and inorganic particulate materials. A large portion of the pulp fines is a cellulosic material that is a homopolysacharide with a sugar moiety that bears free hydroxyl groups available for chemical reaction. They are capable of undergoing the typical reactions known for primary and secondary alcohols. Polymers containing hydroxyl groups can be derivatized to produce materials with unlimited number of applications such as thickeners for foods, coatings, paints, explosive slurries, oil well fluids, cosmetics and other personal care products, and many other functional applications.
Derivatives of polyhydroxyl polymers are usually obtained by chemical reaction of the hydroxyl groups of the monosaccharide moiety with alkylene oxides (ethylene, propylene, butylene or higher oxides) or alkyl chlorides, in the presence of an alkaline catalyst (such as sodium hydroxide). The derivatization reaction may be conducted under heterogeneous conditions in the substantial absence of water or solvent although the efficiency of the reaction is low without the addition of water. Accordingly, the reaction is generally conducted in the presence of small amount of water to provide better reactivity. For instance, carboxymethylhydroxyalkyl derivatives of celluloses are obtained by chemical reaction of the hydroxyl groups of celluloses chain with a mixture of alkylene oxides (ethylene, propylene, butylene or higher oxides) and chloroacetic acid, in the presence of an alkaline catalyst.
There are numerous publications and patents on the preparation and utilization of cellulosic fibers (Gedon & Fengl, 1993; Heinze & Liebert, 2004; Majewicz & Padlas, 1993; Serad, 1993). However, to the best of our knowledge there are no known reports addressing the conversion of pulp fines into a hydrogel forming material in a single step process.